Letter to the Editor

Herschel/HIFI observations of spectrally resolved methylidyne signatures toward the high-mass star-forming core NGC 6334I ^*,**

¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700AV, Groningen, The Netherlands e-mail: wiel@astro.rug.nl
² SRON Netherlands Institute for Space Research, Groningen, NL
³ California Institute of Technology, Pasadena, USA
⁴ Department of Astronomy, University of Michigan, Ann Arbor, USA
⁵ Max-Planck-Institut für Radioastronomie, Bonn, Germany
⁶ Physikalisches Institut, Universität zu Köln, Köln, Germany
⁷ Centre d'Étude Spatiale des Rayonnements, Université Paul Sabatier, Toulouse, France
⁸ CNRS/INSU, UMR 5187, Toulouse, France
⁹ Laboratoire d'Astrophysique de Grenoble, UMR 5571-CNRS, Université Joseph Fourier, Grenoble, France
¹⁰ Université de Bordeaux, Laboratoire d'Astrophysique de Bordeaux, Floirac, France
¹¹ CNRS/INSU, UMR 5804, Floirac Cedex, France
¹² Jet Propulsion Laboratory, Caltech, Pasadena, CA 91109, USA 

¹³ Ohio State University, Columbus, OH, USA
¹⁴ Infared Processing and Analysis Center, Caltech, Pasadena, USA
¹⁵ Johns Hopkins University, Baltimore MD, USA
¹⁶ INAF - Istituto di Fisica dello Spazio Interplanetario, Roma, Italy
¹⁷ Laboratoire d'Études du Rayonnement et de la Matière en Astrophysique, UMR 8112 CNRS/INSU, OP, ENS, UPMC, UCP, Paris, France
¹⁸ School of Physics and Astronomy, University of Leeds, Leeds UK
¹⁹ Centro de Astrobiologìa, CSIC-INTA, Madrid, Spain
²⁰ INAF Osservatorio Astrofisico di Arcetri, Florence, Italy
²¹ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Amsterdam, The Netherlands
²² Department of Astrophysics/IMAPP, Radboud University Nijmegen, Nijmegen, The Netherlands
²³ IGN Observatorio Astronómico Nacional, Alcalá de Henares, Spain
²⁴ Max-Planck-Institut für Astronomie, Heidelberg, Germany
²⁵ Harvard-Smithsonian Center for Astrophysics, Cambridge MA, USA
²⁶ INAF - Osservatorio Astronomico di Roma, Monte Porzio Catone, Italy
²⁷ Institut de RadioAstronomie Millimétrique, Grenoble, France
²⁸ Leiden Observatory, Leiden University, Leiden, The Netherlands
²⁹ Department of Physics and Astronomy, University College London, London, UK
³⁰ Dept. of Physics and Astronomy, University of Waterloo, Canada
³¹ European Space Astronomy Centre, ESA, Madrid, Spain

Received: 31 May 2010
Accepted: 7 July 2010

Abstract

Context. In contrast to the more extensively studied dense star-forming cores, little is known about diffuse gas surrounding star-forming regions.

Aims. We study the molecular gas in the Galactic high-mass star-forming region NGC 6334I, which contains diffuse, quiescent components that are inconspicuous in widely used molecular tracers such as CO.

Methods. We present Herschel/HIFI observations of methylidyne (CH) toward NGC 6334I observed as part of the “Chemical HErschel Survey of Star forming regions” (CHESS) key program. HIFI resolves each of the six hyperfine components of the lowest rotational transition (J = –) of CH, observed in both emission and absorption.

Results. The CH emission features appear close to the systemic velocity of NGC 6334I, while its measured FWHM linewidth of 3 km s^-1 is smaller than previously observed in dense gas tracers such as NH₃ and SiO. The CH abundance in the hot core is ~7 × 10^-11, two to three orders of magnitude lower than in diffuse clouds. While other studies find distinct outflows in, e.g., CO and H₂O toward NGC 6334I, we do not detect any outflow signatures in CH. At least two redshifted components of cold absorbing material must be present at –3.0 and +6.5 km s^-1 to explain the absorption signatures. We derive a CH column density (N_CH) of 7 × 10¹³ and 3 × 10¹³ cm^-2 for these two absorbing clouds. We find evidence of two additional absorbing clouds at +8.0 and 0.0 km s^-1, both with N_CH ≈ 2 × 10¹³ cm^-2. Turbulent linewidths for the four absorption components vary between 1.5 and 5.0 km s^-1 in FWHM. We constrain the physical properties and locations of the clouds by matching our CH absorbers with the absorption signatures seen in other molecular tracers.

Conclusions. In the hot core, molecules such as H₂O and CO trace gas that is heated and dynamically influenced by outflow activity, whereas the CH molecule traces more quiescent material. The four CH absorbing clouds have column densities and turbulent properties that are consistent with those of diffuse clouds: two are located in the direct surroundings of NGC 6334, and two are unrelated foreground clouds. Local density and dynamical effects influence the chemical composition of the physical components of NGC 6334, which causes some components to be seen in CH but not in other tracers, and vice versa.